Claflin D R, Morgan D L, Julian F J
Department of Anesthesia Research Laboratories, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts 02115.
Biophys J. 1989 Nov;56(5):967-77. doi: 10.1016/S0006-3495(89)82742-0.
Experiments were performed to determine the influence of sarcomere length and passive tension on the velocity of unloaded shortening (Vu) as measured by the slack test technique. Slack test results were obtained from intact twitch fibers isolated from the frog (Rana temporaria). Measurements were made both in the absence and presence of passive tension using two different protocols. In one, all releases were initiated from the same sarcomere length and passive tension level; in the other, all releases ended at the same sarcomere length. In the absence of passive tension, no difference was observed between the results from the two slack test protocols. When passive tension was present, performing all releases from the same initial sarcomere length and passive tension level resulted in linear step size-slack time relationships in which the slopes (Vu) were independent of length over a sarcomere length range extending to 3.1 microns, and the intercepts increased with increasing sarcomere length. Performing all releases to the same final sarcomere length in the presence of passive tension produced nonlinear step size-slack time relationships. The results presented here show that, in the presence of significant levels of passive tension, the traditional interpretation of the slope of the slack test plot as the constant unloaded shortening velocity is only correct when all length steps are initiated from the same initial sarcomere length and level of passive tension.
进行了实验，以确定肌节长度和被动张力对通过松弛测试技术测量的无负荷缩短速度（Vu）的影响。松弛测试结果取自从青蛙（欧洲林蛙）分离出的完整单收缩纤维。使用两种不同的方案在有无被动张力的情况下进行测量。在一种方案中，所有释放均从相同的肌节长度和被动张力水平开始；在另一种方案中，所有释放均在相同的肌节长度结束。在没有被动张力的情况下，两种松弛测试方案的结果之间未观察到差异。当存在被动张力时，从相同的初始肌节长度和被动张力水平进行所有释放，会产生线性步长-松弛时间关系，其中斜率（Vu）在延伸至3.1微米的肌节长度范围内与长度无关，且截距随肌节长度增加而增大。在存在被动张力的情况下，将所有释放进行到相同的最终肌节长度会产生非线性步长-松弛时间关系。此处给出的结果表明，在存在显著水平的被动张力时，只有当所有长度步长均从相同的初始肌节长度和被动张力水平开始时，将松弛测试图的斜率传统解释为恒定的无负荷缩短速度才是正确的。
